The paper presents the results of experimental tests carried out on steel sample with the scope to verify the influence of planar defect with different size on type of fracture. The sample have been pressed up to the collapse. During the loading the sample have been monitored with acoustic emission to pick up the behavior of the material subject to increasing loading. The result of the tests highlights interesting data on the change of the type of fracture as the size of defects increase. From ductile fracture with 10% reduction of resistant section to brittle fracture with 30% reduction of resistant area. This change in the structure may be interpreted due to the hardening of the section consequently the increasing of the stress. This explanation is coherent with the string theory model of the metallic structure. According this model the hardening of material is consequence of any process that introduce stress inside the structure. Interesting the information coming from the acoustic emission maps which underline the different behaviour of the material with increasing stress.